In recent years both governmental and private agencies have stressed the damage which can result when eyes are exposed to bright sunlight, this damage being caused primarily by exposure to wavelengths in the ultraviolet portion of the radiation spectrum. Therefore, glass manufacturers have been active in designing compositions for sunglasses which will essentially block the transmittance of ultraviolet radiation. U.S. Pat. No. 5,256,607 (Kerko et al.) and U.S. Pat. No. 5,268,335 (Kerko et al.) present two disclosures directed to the production of such glasses.
U.S. Pat. No. 5,256,607 describes glass compositions suitable for use as sunglasses which exhibit a neutral gray fixed tint coloration and limit the transmittance of ultraviolet radiation at a wavelength of 380 nm to no more than 1% at a thickness of 2 mm. The glass compositions are preferably essentially free of ZnO and consist essentially, expressed in terms of weight percent on the oxide basis, of
______________________________________ SiO.sub.2 65-72 K.sub.2 O:Na.sub.2 O 1.25-2.25 As.sub.2 O.sub.3 0-0.3 B.sub.2 O.sub.3 2-6 Al.sub.2 O.sub.3 0-2.25 Fe.sub.2 O.sub.3 4.8-6.2 Na.sub.2 O 6-10 CaO 0-1.5 Co.sub.3 O.sub.4 0.012-0.02 K.sub.2 O 10-16 Al.sub.2 O.sub.3 + Cao 0-1.5 NiO 0.16-0.21. Na.sub.2 O + 17-23 K.sub.2 O ______________________________________
The high concentration of iron provided the required absorption of ultraviolet radiation. Cobalt and nickel were added to adjust the chromaticity of the glass to a neutral target. Al.sub.2 O.sub.3, B.sub.2 O.sub.3, and/or CaO were included to assist in suppressing the transmittance of the glass at a wavelength of 380 nm to below 1% at a thickness of 2 mm.
U.S. Pat. No. 5,268,335 discloses the fabrication of ophthalmic lenses which can be chemically strengthened to high values with surface compression layers of substantial depth through short treatments and which restrict the transmittance of ultraviolet radiation at 380 nm to no more than 1% at a thickness of 2 mm. Iron was utilized to achieve the demanded low transmittance of ultraviolet absorption and, where a neutral gray coloration was desired, cobalt and nickel were included. The base compositions therefor were preferably free of ZnO and consisted essentially, expressed in terms of weight percent on the oxide basis, of
__________________________________________________________________________ SiO.sub.2 55-65 Na.sub.2 O 6-18 TiO.sub.2 0-4 B.sub.2 O.sub.3 5-20 K.sub.2 O 2-10 ZrO.sub.2 0-7 Al.sub.2 O.sub.3 4-10 Li.sub.2 O + Na.sub.2 O + K.sub.2 O 13-22 MgO + TiO.sub.2 + ZrO.sub.2 0-10 B.sub.2 O.sub.3 + Al.sub.2 O 14-26 CaO 0-1.5 As.sub.2 O.sub.3 0-0.5 Li.sub.2 O 0-3 MgO 0-4 ZnO 0-1.5. __________________________________________________________________________
Where other tints are desired in the glass, the patent cited the use of Co.sub.3 O.sub.4, Cr.sub.2 O.sub.3, CuO, Fe.sub.2 O.sub.3, MnO.sub.2, NiO, V.sub.2 O.sub.5, Au, Pd, and rare earth metal oxides.
The patent also disclosed a glass commercially marketed by Corning Incorporated, Corning, N.Y., which exhibits a neutral gray fixed tint and limits the transmittance of ultraviolet radiation at 380 nm to no more than 1% at a thickness of 2 mm. An analysis of that glass, marketed under Corning Code 8015, is tabulated below expressed in terms of weight percent on the oxide basis:
______________________________________ SiO.sub.2 68.41 K.sub.2 O 9.71 Co.sub.3 O.sub.4 0.021 Al.sub.2 O.sub.3 0.51 ZnO 6.76 NiO 0.126 Na.sub.2 O 8.81 Fe.sub.2 O.sub.3 5.54 As.sub.2 O.sub.3 0.111. ______________________________________
There has been a need for a glass suitable for ophthalmic applications exhibiting a brown fixed tint of a desirable hue that can be chemically strengthened to high values with surface compression layers of significant depth, and which limits the transmittance of ultraviolet radiation at 380 nm to no more than 1% at a thickness of 2 mm.
Fixed tint brown ophthalmic glasses which are chemically strengthenable are known in the art. Code 8079 glass, marketed by Corning Incorporated, is illustrative of such. An analysis thereof is set out below, expressed in terms of weight percent on the oxide basis.
______________________________________ SiO.sub.2 65.4 CaO 6.0 NiO 0.48 Al.sub.2 O.sub.3 7.0 ZnO 3.0 V.sub.2 O.sub.5 0.40 Na.sub.2 O 13.9 TiO.sub.2 0.25 As.sub.2 O.sub.3 0.10 K.sub.2 O 2.6 Fe.sub.2 O.sub.3 0.28 Sb.sub.2 O.sub.3 0.60. ______________________________________
The brown color is imparted to the glass through a combination of a relatively high concentration of NiO with a low level of Fe.sub.2 O.sub.3. Unfortunately, however, its transmittance of ultraviolet radiation at a wavelength of 380 nm is greater than 30%.
A fixed tint brown ophthalmic glass which absorbs strongly in the ultraviolet region of the radiation spectrum is marketed by Bausch and Lomb, Inc., Rochester, N.Y., under the designation B-15. That glass has the following approximate composition, expressed in terms of weight percent on the oxide basis:
______________________________________ SiO.sub.2 69.1 K.sub.2 O 9.8 NiO 0.11 Al.sub.2 O.sub.3 0.49 ZnO 6.1 Se 0.046 Na.sub.2 O 8.2 Fe.sub.2 O.sub.3 5.8 As.sub.2 O.sub.3 0.40 ______________________________________
Because the strain point of that glass is so low, viz., about 435.degree. C., it encounters problems in chemical strengthening in the standard commercial white crown glass salt bath; i.e., immersion for 2 to 16 hours in a bath of molten KNO.sub.3 operating at a temperature of 450.degree. C. For effective chemical strengthening brought about by exchanging potassium ions from the salt bath with sodium ions in the glass surface, the strain point of the glass must be higher than the temperature of the salt bath; in this instance higher than 450.degree. C.
Therefore, the principal objective of the present invention was to devise glass compositions suitable for ophthalmic applications which exhibit a brown fixed tint, which exhibit a transmittance to ultraviolet radiation at a wavelength of 380 nm of less than 1%, which will exhibit a transmittance having a dominant wavelength between 580-588 nm with a purity of 48-70%, which exhibit a refractive index of 1.523, and which can be chemically strengthened.